1. Field of the Invention
This invention relates to thermoplastic matrix syntactic foam plugs for use in plug assist thermoforming and the process of making and using the same. In particular the present invention relates to forming syntactic foam plugs from thermoplastic matrix material having a melting temperature and/or glass transition temperature at least 5° C. higher than the operating design temperature of the thermoforming process.
2. Background Art
A plug, or plug assist, is a male tool used in the art of plug assist thermoforming which begins stretching a molten web of material into a females cavity of a forming tool. Traditionally the plug assist has been made of wood, metal, solid plastic, etc., but each has significant disadvantages. Primarily each of these suffer from their relatively high thermal conductivity which removes heat from the molten web and thus adversely affects the stretching of the molten web to be formed into a high quality part. Examples of such adverse affects would include, uneven draw-down (variations in appearance and wall thickness), surface roughness and haziness. These deficiencies may be at least partially overcome through the use of internal or external heating of the plug or employing a multi-component plug but this adds complexity to the device, to the operation and increases the operating cost.
For this reason the industry has found advantages to using syntactic foam materials, or also referred to herein as syntactic material. These are foams formed by incorporating pre-formed hollow particles in a resin matrix. Examples of these hollow particles would include glass or ceramic hollow microspheres. Although there are prior art references directed to syntactic foams, to the best of our knowledge only thermoset (e.g. epoxy and polyester) matrices have been used for making plug assists. This is presumably due to the easy manner in which such foams can be made. The low specific heat and low thermal conductivity of these syntactic foams has been found to resolve many of the recognized processing and economic problems associated with solid plug assist tools. However, these plug assists lack the mechanical toughness desired for the application. They are easily damaged during the operation or during the frequent tool changes typical of their usage and once a dent or chip occurs in a critical area, the tool is unacceptable and must be reworked or scrapped. In addition, it is difficult to machine a brittle epoxy form. The thermoset syntactic foam can chip during machining and the removed materials turn to dust. The dust created leads to special set-ups, and the high potential for the syntactic material including the hollow micro-spheres getting into bearings, etc. Once this occurs, machine wear issues are created. Also, most often the machinist must wear protective equipment to avoid dust inhalation and spend additional time to clean up the area after machining.
Although there are recognized advantages and disadvantages to using syntactic materials for plugs, it was believed that the relatively high operating temperature demands of thermoforming operations limited the selection of syntactic materials to thermoset matrix syntactic materials. As noted in international patent application WO 89/00100, which is hereby incorporated by reference, the shortcomings of these inherently brittle thermoset syntactic materials were addressed by coating the syntactic plug with an elastomeric coating.